1. Field of the Invention
The present invention relates to an organic electroluminescent display device and a method to fabricate the organic electroluminescent display device, more particularly, to an organic electroluminescent display device in which an existing emitting layer is improved to have effective characteristics at high temperature, solve contraction phenomena of pixels and have effective, stable driving characteristics, and a method to fabricate the organic electroluminescent display device.
2. Description of the Related Art
Recently, an organic electroluminescent display device has been proposed as a next generation display device since the organic electroluminescent display device has merits of a thin thickness, a wide viewing angle, light weight, a small size, a fast response speed and lower power consumption compared to a cathode ray tube (CRT) or a liquid crystal display device (LCD). Particularly, the organic electroluminescent display device has an advantage in that it is easily fabricated through a simple fabrication process, since the organic electroluminescent display device is constructed in a simple structure of an anode, an organic layer and a cathode. The organic layer may be formed in various layers according to its functions and generally comprises a hole injection layer, a hole transfer layer, an emitting layer, an electron transport layer, and an electron injection layer.
FIG. 1 is a cross sectional view for schematically showing a structure of a conventional organic electroluminescent display device.
Referring to FIG. 1, holes are injected from an anode 7 that is a transparent electrode so that injected holes are transferred to an emitting layer 4 through hole injection layer 6 and hole transfer layer 5, and electrons are injected from a cathode 1 so that the injected electrons are transferred to the emitting layer 4 through the electron injection layer 2 and the electron transfer layer 3. The transferred electrons and the holes combine with each other to emit light. The emitting layer 4 is constructed so that dopant is doped on a host and the electrons and the holes are transferred to the dopant through the host to emit light.
A phosphorescent material, including iridium (Ir) or platinum (Pt), is used as a dopant in the case of a phosphorescent organic electroluminescent display device, while an organic fluorescent material is used as an emitting material in the case of a fluorescent organic electroluminescent display device.
The method of the present invention improves the emitting layer to improve the stability of an organic electroluminescent display device at a high temperature. The life cycle of the organic electroluminescent display device at a high temperature is improved by forming a host of the emitting layer using a mixture of the hole transport layer and the electron transport layer to improve high temperature stability of an organic electroluminescent display device, as is disclosed in U.S. Pat. No. 6,392,339. The stability of the organic electroluminescent display device at a high temperature is improved by suppressing the movement of the holes to the electron transport layer using an emitting layer comprising a mixture of the hole transport layer and the electron transport layer. The driving stability of an organic electroluminescent display device is improved by using a mixture of the hole transport layer and the electron transport layer as a host of the emitting layer and doping a dopant, as is disclosed in U.S. Pat. No. 6,392,250.
However, it has been confirmed that when these methods are applied to a fluorescent material so that the stability of an organic electroluminescent display device is improved at a high temperature, the methods rather reduce the characteristics of the device in the case of a phosphorescent organic electroluminescent display device. Particularly, these methods have a limit in improving characteristics of the device at a high temperature since a host material having electron transportability is limited in the phosphorescent organic electroluminescent display device. Furthermore, it is necessary to improve characteristics of the host of the emitting layer, as well as the characteristics of the dopant, to improve the characteristics of an organic electroluminescent display device.